1. Field of the Invention
The present invention relates generally to an apparatus and method for allocating resources in a wireless communication system. More particularly, the present invention relates to an apparatus and method for allocating and releasing downlink resources.
2. Description of the Related Art
Many technologies have been proposed as candidates for high-speed wireless mobile communications. Among them, Orthogonal Frequency Division Multiplexing (OFDM) is considered the most promising future-generation wireless communication technology. OFDM has been adopted as a standard for Institute of Electrical and Electronics Engineers (IEEE) 802.16 Wireless Metropolitan Area Network (WMAN) and it is expected that OFDM will be adopted for most wireless communication by 2010.
In an OFDM-Broadband Wireless Access (BWA) system, a Base Station (BS) transmits information describing downlink/uplink resource allocation in every frame in order to configure the downlink/uplink effectively and freely. The information is delivered by a MAP message in an IEEE 802.16 system.
For downlink data transmission, the BS allocates downlink resources to a destination Mobile Station (MS) and transmits a DownLink-MAP (DL-MAP) Information Element (IE) with resource allocation information to the MS. The downlink resources are two-dimensional. For example, they take the form of symbols and subchannels. A symbol is defined in time and a subchannel is defined in frequency. In general, a basic unit of downlink resource allocation is a slot and the amount of resources is represented as the number of slots. The MS can receive downlink data from the BS in the allocated resources.
FIG. 1 illustrates a conventional downlink resource allocation method in a wireless communication system.
A frame includes a DL frame and an UpLink (UL) frame. The DL frame carries data from a BS to MSs and the UL frame carries data from the MSs to the BS in predetermined areas. The DL frame includes a preamble, a Frame Control Header (FCH), a DL-MAP, a UL-MAP, and DL data bursts. The preamble is used for initial synchronization and cell search in the MSs and the FCH provides information describing the basic configuration of the frame. The DL-MAP includes information indicating DL data burst areas and the UL-MAP includes information describing the configuration of the UL frame.
As described above, the MAP messages with resource allocation information are transmitted in every frame because a plurality of MSs reside in a cell and have mobility. In other words, because the MSs are in time-varying channel statuses, the BS should schedule resources for the MSs and notify them of the scheduling results in every frame.
The MSs obtain knowledge of the frame configuration and the allocated resources by receiving the DL-MAP and the UL-MAP in every frame from the BS. MAP IEs in the DL-MAP and the UL-MAP are mapped to data bursts in a one-to-one correspondence.
Referring to FIG. 1, a BS allocates 24 DL slots, 8 DL slots, 9 DL slots, and 10 DL slots in the form of rectangles to four MSs, respectively by four DL-MAP IEs. Each DL-MAP IE includes information about the number of allocated slots. Each of the MSs obtains knowledge of resources allocated to the other MSs by sequentially interpreting the DL-MAP IEs, finds the start of resources allocated to the MS, and counts slots as indicated by the slot number, starting from the start to thereby determine the allocated resources for the MS. The DL-MAP IE includes a Connection Identification (CID) that identifies a service and an MS, an Uplink Interval Usage Code (UIUC) indicating a modulation scheme, an offset indicating the start of allocated slots, a duration indicating the number of allocated slots, and a repetition coding indication that indicates the number of iterative codings.
If the BS is to allocate the same downlink resources to the four MSs in each frame, it should broadcast the four DL-MAP IEs illustrated in FIG. 1 in each frame. For example, in the case of a service that transmits downlink data periodically (e.g. Voice over Internet Protocol (VoIP)), the BS should broadcast DL-MAP IEs in every frame, resulting in the unnecessary consumption of resources.
The IEEE 802.16 system allows maximal freedom in frame configuration by defining a minimum transmission unit (data burst) by one subchannel and one symbol on the downlink. However, such higher freedom requires a larger amount of control information. If data for a plurality of users coexist in one frame, information that is delivered to the users by a DL-MAP and a UL-MAP imposes a large overhead. In the worst case scenario, only a very small amount of resources are available for actual traffic and thus system throughput is decreased.